How do you restore someone else's old iPhone?

I have some of my family’s old 2G iPhones here and I was going to trade them in to Amazon. I’m trying to wipe them out, but when I plug them into iTunes, it wants me to unlock the iPhone using the passcode. The trouble is, we haven’t used the phones in years, and I’m not even sure if we have the computers they synced with back then. Is there a way to completely wipe them out without knowing the passcode or having the computer it was paired with?